U.S. Pat. No. 2,947,617 describes cubic boron nitride and its preparation by a catalytic, high pressure/high temperature (HP/HT) technique. Other U.S. patents on the subject of the preparation of CBN are: U.S. Pat. Nos. 3,150,929; 3,192,015; 3,701,826; 3,918,931; and 3,959,443. Some patents directed specifically toward the conversion of hexagonal boron nitride (HBN) to cubic boron nitride without a catalyst present are: U.S. Pat. Nos. 3,212,852 and 4,150,098 and British Pat. Nos. 1,317,716 and 1,513,990. Also U.S. Pat. No. 4,289,503 incorporated by reference herein discloses an improved process for converting hexagonal boron nitride to cubic boron nitride and also discloses aggregated grinding grits manufactured by that improved process.
One of the methods for making the aggregated cubic boron nitride abrasive described above in U.S. Pat. No. 4,289,503 comprises the following steps:
(a) a vacuum firing pre-treatment step in which hexagonal boron nitride powder is held at a temperature of about 1400.degree.-1900.degree. C. for a time of about 5 minutes to 4 hours and at an initial pressure of about 10.sup.-3 to 10.sup.-10 mm. Hg. (which would increase to greater than 10.sup.-3 mm.Hg during heating due to nitrogen gas evolution);
(b) mixing the resulting powder with single-crystal CBN particles having a maximum dimension ranging from 5 to 150 microns and in a concentration of 5-30 volume percent single-crystal CBN;
(c) prepressing the mixture from part (b) in a hand press at about 20,000 psi (137.9 kPa);
(d) subjecting the prepressed mixture to the HP/HT process: (i) at a pressure of 55-80 kilobars (preferably 65-75 kbar), (ii) at a temperature of from 1600.degree. C. to the reconversion temperature of cubic boron nitride (preferably 2000.degree.-2300.degree. C.), (iii) for a time sufficient to convert the hexagonal boron nitride to cubic boron nitride and sinter the cubic boron nitride (about 8 minutes), and (iv) in the absence of catalyst and impurities; and
(e) recovering the CBN.
In recovering the CBN from the high pressure apparatus, most of the high pressure reaction cell material is physically removed, leaving relatively large pieces of the product specimen with carbon, possibly shield metal and other cell materials present. These pieces are treated with a mixture of sulfuric and nitric acids to remove residual carbon and metal impurities. The undissolved solids are washed in water and then mixed with a mixture of nitric and hydrofluoric acids to dissolve any remaining shield metal and gasket materials from the high pressure cell. This step is followed by a final water wash of the CBN pieces. Large lumps of the CBN may be impact milled to powder, size separated, and ultrasonically cleaned to yield the desired aggregated grit.
Reconversion temperature is defined as that temperature at which boron nitride reconverts from the cubic crystal structure to the hexagonal. This temperature is found along the equilibrium line separating the hexagonal boron nitride stable region from the cubic boron nitride stable region in the phase diagram for boron nitride (see U.S. Pat. No. 3,212,852; FIG. 6 and Column 8, line 66-Column 9, line 42).
The purpose of the vacuum firing is two-fold, first, to remove boric oxide from the surface of the hexagonal boron nitride powder, and secondly, to generate a coating of boron on the surfaces of the powder particles. In order to accomplish this second purpose, it is necessary to carry out the vacuum firing in the boron nitride thermal decomposition range. The relative amount of the free boron developed can be inferred visually from the discoloration of the vacuum fired powder. At the lower firing temperatures (1500.degree.-1650.degree. C.) where only a slight amount of surface boron is generated, vacuum fired powder has a light reddish brown tint. The depth of color increases with increasing firing temperature or time until at the higher firing temperatures (1800.degree.-1900.degree. C.) the particles are covered with a black boron surface coating.
It is not absolutely necessary that the single crystal CBN inclusions of step (b) be used. The above described process can be used with no such inclusions or with other types of inclusions, such as refractory metal powder, so long as the inclusion material does not interfere with the high pressure conversion of hexagonal boron nitride to CBN.
The vacuum fired hexagonal boron nitride converts in the HP/HT process to a polycrystalline material which may be dulled by attritious wear. The alternative embodiments in which the single crystal cubic boron nitride (or other inclusions) are mixed with the vacuum fired powder prior to high pressure/high temperature processing are preferred because they seem to result in an aggregate particle (i.e. polycrystal containing single crystal or other inclusions) having breakdown characteristics which makes it advantageous for use in some grinding applications.
U.S. Pat. No. 3,852,078 discloses bonded CBN bodies in which hexagaonal boron nitride is mixed with CBN before high pressure/high temperature processing, but no pre-treatment of hexagonal boron nitride is required.
The hexagonal boron nitride utilized in the above-described process is ideal hexagonal or graphitic boron nitride (GBN). Two forms of hexagonal boron nitride have been identified, turbostratic and graphitic. The turbostratic structure is characteristic of pyrolytic boron nitride and is a continuous structure characterized by two-dimensional layers of hexagonal rings stacked at irregular intervals and randomly oriented. GBN generally has a more ordered crystal structure than turbostratic or pyrolytic boron nitride. The boron and nitrogen atoms are believed to form more or less parallel stacks of fused BN layers in the hexagonal lattice, with the stacking being fairly ordered in translation parallel to the layers and also in rotation about the normal to the layers. In other words, there are fewer imperfections and distortions within the GBN structure. GBN has a density of about 2.28 g/cm.sup.3 and an interlayer spacing of about 3.33 angstroms. The structure in any mass of GBN is continuous in any given direction, as opposed to being separated by crystal boundaries. The material is generally soft, flaky and light in color.
Further details on the two forms of hexagonal boron nitride may be found in Thomas, J. et al., "Turbostratic Boron Nitride, Thermal Transformations to Ordered-layer-lattice Boron Nitride", J. A. C. S., Vol. 84, (Jan. 25, 1963) p.4619; and Economy, J. and Anderson, R., "Boron Nitride Fibers", J. Polymer Science: Part C, No. 19, (1967) p. 283.
In the HP/HT process step (d) the pressure and then the temperature are increased and held at the desired conditions for the desired time. The sintered sample is allowed to cool under pressure for a short period of time, and then the pressure is decreased to atmospheric. The mass of polycrystalline cubic boron nitride is then recovered. Care must be exercised in the design of the high pressure cell to ensure against impurity penetration from exterior cell parts into the sample.
Because of the boron coating generated during the pre-treatment step (a) the cubic boron nitride resulting from the above-described process (usually in the form of grinding grit) is itself boron rich. By following the teachings of British Pat. No. 1,513,990 (e.g., incorporating boron powder, aluminum boride, or mixtures of aluminum and boron into HBN in an HP/HT process) one also obtains boron rich CBN. In order to test the performance of the boron rich abrasives in plated tools (e.g. nickel plated grinding wheels) an attempt was made to fabricate a nickel plated wheel. Severe overplating of the abrasive grains occurred which prohibited testing of the wheel and which would prohibit use of the boron rich abrasives in plated tool applications. The invention disclosed herein presents a solution to the severe overplating problem. A discussion of plating problems and solutions for diamond tools may be found in Pope, B. J. and Stark, P., "Synthetic Diamond for Plated Products", In Proceedings: "Diamond in the 80's" A Technical Symposium of Industrial Diamond Assn. of America, Chicago, Ill., Oct. 13-15, 1980, pp. 113-126.